Stereo rangefinder

ABSTRACT

A photographic camera having a variable focus objective lens incorporates a binocular rangefinder of the &#39;&#39;&#39;&#39;stereo&#39;&#39;&#39;&#39; type including a reticle and lens means for projecting a virtual image of the reticle a predetermined distance into the viewed field. Left and right eye view windows are located to intercept different portions of common imaging wavefronts defining the virtual reticle image. The rangefinder is coupled to a focus adjustment mechanism for the camera such that the image distance of the projected reticle image and the focused distance of the camera objective lens are caused to vary in correspondence. A novel rangefinding method is also disclosed.

United States Patent Norris [4 1 May 9, 1972 [54] STEREO RANGEFINDERPrimary [Stamina-Samuel S. Matthews Assistant Examiner-Richard A.Wintercorn [72] Inventor: Philip R. Norris, North Readmg Massp AmmeyBrown and Mikulka [73] Assignee: PolaroidCorporntion, Cambridge, Mass.221 Filed: May 15, 1970 i [57] I ABSTRACT A photographic camera having avariable focus objective lens [211 App! 37667 incorporates a binocularrangefinde'r of the stereo" type including a reticle and lens means forprojecting a virtual image [52] 0.8. CI. ..95/44 C, 356/8, 356/12 of thereticle a predetermined distance into the viewed field. Left and righteye view windows are located to intercept dif- [58] Field of Search--95/4 44 C; 12 ferent portions of common imaging wavefronts definingthe v virtual reticle image. The rangefinder is coupled to a'focus ad-[56] Cited justment mechanism for the camera such that the image 1 UNTEDSTATES PATENTS distance of the projected reticle image and the focuseddistance of the camera ob ective lensare caused to vary in 3.200.727 5Bart n a 12 X correspondence. A novel rangefinding method is alsodisclosed.

22 Claims, 7 Drawing Figures ,R a L I? \L IB J 7' it g s 23 $3111 i J RI 30 1 C9 26 2B 27 Q: L 29 I n l I I a 24 j R 25 v 7 .PATENTEDKAY 9:912

,R lz Z as Q) INVENTOR PHILIP R. NORRIS ATTORNEYS 'PATENTEDIAY 9 I972INVENTOR PHIL IP R. NQRRIS BY amwmwao and 194: 766. M ATTORNEYSPAIENTEDMAX 91912 4 3,661,064

smile or 3 WWW I INVENTOR PHILIP R. NORRIS 4 ATTORNEYS srnnnonnncnmnmBACKGROUND OF THE INVENTIONv This application concems. a photographiccamera having a binocular rangefinder of the type commonly termed stereorangefinder" wherein a virtual image of a reticle is projected avariable distance into the field.

Stereo rangefinders of the described type have traditionally employed apair of like left and right eye telescopes. For example, see US. Pat.Nos. 1,514,948 Baer et al.; 2,144,257 Eppenstein; and 2,425,713Applegate. Each telescope includes a reticle (or erects, a reticleimage) at or within the focal plane of the telescope: objective; virtualimages of the reticles are projected'into the field by. the objectives,As evidenced by the noted patents, these telescopes usually havesubstantial optical complexity. v

Various mechanisms, many of them quite intricate, have been employed foradjustingthe depth-wise position of the respective reticle images in.the field and for simultaneously varying the angle of convergenceoftheleft and right projection axes in order to cause the convergencepoint of'the axes and the reticle images to coincide and to move incorrespondence.

The attendant alignment, calibration, and correlation problemsencounteredin attempting to achieve the above are substantial. Variousattempts to overcome certain of these problems are illustrated in US.Pat. Nos. 2,166,046. French; 2,350,210 Aulin; and 2,991,683 Gunther,inter alia. Compensation for inter-ocular variations among users is alsodifficult in such dual systems, as recognized in Pat. No. 2,932,234Gunther.

OBJECTS OF THE INVENTION It is a primary object of this invention toprovide an improved rangefinding method and to provide a stereorangefinder in which exact registration of the virtual left and righteye rticle images perceived byan observer is inherent for any and allpositions of the reticle image in the field. It is thus an object toprovide a stereo ran'gefinder in which problems associated withstructural alignment and calibration, registration of "images,correlation of image movements, and sensitivity to inter-ocularvariations are substantially non-existent.

It is yet'another object of thisinvention to provide a stereorangefinder which .is characterized byextreme simplicity in structureand operation.

Itis a further object to provide a photographic camera having a coupledrangefinder in which a projected reticle image is caused to advanceorrecede in the field in response to adjustments of a focus controlmechanism for the camera objective.

Further objects and advantages of the invention will in part be obviousand will in part become apparent as the following description proceeds.The features of novelty which characterize the invention will be pointedout with particularity in the claims annexed to and forming a part ofthis specification.

BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding of theinvention, reference may be had to the following detailed descriptiontaken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic perspective view of a photographic camera having acoupled rangefinder' constructed in accordance with the teachings ofthis invention;

FIG. 2 is a top (plan) view of a-portion of the rangefinder shown inFIG. 1;

FIG. 3 is an alternative embodiment of reticle means shown in FIG. I v

FIG. 4 is an extremely schematic perspective view of a second embodimentof the invention which is particularly useful in illustrating theprinciples of this invention;

FIGS. 5 and 6 are side and front elevational views of the FIG. 4embodiment; and I FIG. 7 is a view of the FIGS. 4-6 rangefinder adjustedsuch that the projected reticle image has been advanced toward theobserver.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 illustrate aphotographic camera incorporating a stereo rangefinder representing apreferred implementation of the principles of this invention. FIGS. 4-7show a second embodiment of the invention which is perhaps more usefulthan the FIGS. 1-2 embodiment in clarifying the underlying principles,of this invention. In order that the invention may be clearly understoodbefore a discussion of FIGS. l-2 is engaged, the FIGS. 4-7 embodimentwill be discussed first.

As noted above, all known, prior art stereo rangefinders employ a pairof substantiallyoptically independent reticle projection systems.Because of the independenceof the left and right projection systems,there exists serious problems in attempting to register the. left andright reticle images in the field, and of maintaining close registrationof these images as the effective projected distance of the compositestereo reticle image is caused to. recede or advance in the field.

At the heart of this invention lies the fact that a single reticle and asingle imaging meansare employed to project a single virtual image ofthe reticle into the field. The image is viewed by the left and righteyes of an observer which intercept different portions of common imagingwavefronts constituting the reticle image. In other words, both eyesview a single reti-, cle image by intercepting difiierent portions of asingle bundle of imaging rays which define the projected virtual reticleimage. Thus, this invention provides for the first time a truestereoscopic rangefinder in which the left and right eyes of theobserver receive a true stereo pair of images of a single unitaryreticle image, rather than receiving a simulated stereo pair of imageswhich, when properly viewed, appear to represent a single unitaryreticle image. v

By the fact that a single reticle image is erected, rather than a pairof images as in prior art approaches, the problems of misregistration,correlation of changing left and right image distances, and tracking ofthe convergence point of a pair of stereo axes by the left and rightreticle images are all obviated.

FIGS. 4-7 show a reticle 8 and a dioptric lens 9, located at most afocal length away from the reticle 8, for forming a virtual image 100ithe reticle 8. A partially reflective planar mirror 11 is angled withrespect to the optical axis 0-0 of the lens 9 to reflect the virtualreticle imageIO into the field of view of an observer. The observer isrepresented in FIGS. 4 and 7 by a left eye LE and a right eye RE. As canbe seen quite clearly in FIGS. 4-6, theleft and right eyes of theobserver intercept left and right portions 121., 12R of a single bundle13 of imaging rays which define the reticle image 10. Stated in terms ofwaves rather than rays, the observer's left and right eyes interceptdifferent portions of common imaging wavefronts defining the virtualreticle image. 7

FIG. 7 is a view very similar to FIG. 4 but having the object distancefrom the reticle 8 to the lens 9 reduced so as to effect a correspondingdecrease in the image distance of the reticle image 10. It is evidentfrom FIGS. 4-7 that as the virtual reticle image 10 is caused to advanceor recede in the field, the observer is able to follow the image 10 inthe manner in which he would follow any object moving toward or awayfrom him in the field.

FIGS. 1 and 2 represent a structurally more preferred embodiment of theprinciples of this invention. A camera is shown very schematically asincluding a variable focus lens 14 for forming images on an element ofphotosensitive material 15. Means for controlling the exposure intervaland effective aperture are not shown, but may be of any suitable commer-In the same manner that dioptric lens 9 in the FIGS. 47 em bodimentprojects a virtual image of reticle 8, the catoptric lens 18 projectsinto the field a virtual image 22 of the first reticle image 21 formedby mirror 20. The projected reticle image 22 may be seen by the left andright eyes of an observer located at left and right eye view windows 23,24 in the rangefinder housing 25 and looking along left and right ocularaxes L-L and R-R, respectively.

As explained with reference to the FIGS. 4-7 embodiment, the left andright eyes of an observer located at the left and right eye windows 23,24 intercept different portions of common imaging wavefronts (formed bythe catoptric lens 20) which define the projected reticle image 22. Theimage distance of the projected reticle image 22 is identifiable withthe range of objects in the field which appear, to an observer lookinginto the field through the windows 23, 24, to be situated in thevicinity of the reticle image 22.

In order that the position of the projected reticle image 22 may bevaried along the axis 0,0, i.e., in a depth-wise sense, means areprovided for adjusting the axial position of the mirror 20 relative tolens 18 and thus the object distance of the first reticle image 21formed by the mirror 20. It is an object of this invention to provide acamera having a coupled stereo rangefinder in which movement of theprojected reticle image is correlated with adjustments in the focuseddistance of the camera objective.

To these ends, in order to couple movement of the mirror 20 toadjustments of the focus of objective lens 14, coupling means areprovided. In the illustrated embodiment, the lens assembly containingthe lens 14 is of the type wherein rotation of a lens barrel 26 causesthe barrel 26 and the lens 14 contained therein to move along the axisthereof. The lens barrel 26 is shown as including acam ring 27. Acoupling member 28 has a cam follower 29 which engages the cam ring 27at one end and at the other end is connected directly to support means30 holding the mirror 20. Rotation of the lens barrel 26 to adjust thefocus of the lens 14 causes the mirror 20 to move in a correspondingdirection. It is manifest from an inspection of FIGS. 1 and 2 that aforward movement of the lens 14 so as to focus on nearer objects resultsin a forward movement of the mirror 20 and a resultant advancement ofthe projected reticle image 22 toward the observer. The profile of thecam ring 27 is selected such as to cause the projected reticle image 22to track (follow) the focused distance of the lens 13.

An important aspect of this invention is the extreme simplicity of therangefinders disclosed. By the use of a single reticle which is firstimaged virtually by a position-adjustable mirror and then reimaged intothe field by imaging means (such as the described catoptric lens), theoptical apparatus necessary to erect a stereoscopically observablereticle image is very greatly simplified. Further, the capability ofmoving the projected image while maintaining inherent registration ofleft and right eye images by merely adjusting the position of a singleelement, the adjustable mirror, is a vast and significant advancementover prior art structures. Thus, this invention teaches rangefindingmethods and apparatus of extreme simplicity in which left and right eyeimages are inherently registered and in which depth-wise movement of theprojected image is accomplished by adjustment of a single opticalelement and during which movement image registration is inherentlymaintained. This invention makes possible for the first time a low coststereo rangefinder of uncompromising accuracy and ease of use. I

FIG. 3 illustrates a three-dimensional reticle 32 which may besubstituted for the reticle 17 in the FIGS. 1-2 embodiment, or may beadapted to be substituted for the reticle 8 in the FIGS. 4-7 embodiment.A virtual image of the reticle 32 will have an illusion of depth due tothe depth-wise extension of the reticle 32, the smaller arrow 34appearing more distant from the observer than the larger arrow 36.

The reticle 32 may be constructed by forming the smaller arrow 34 as afrosted light-transmitting window in a mask 38. The arrow 36 maycomprise a frosted light-transmitting window defined by mask 38 and amasked rear surface on a spacer plate 40. The arrows 34, 36 will thus beaxially separated in space to cause the images of the arrows formed inthe field to also be spaced in a depth-wise sense.

The invention is not limited to the particular details of constructionof the embodiments depicted, and it is contemplated that various andother modifications and applications will occur to those skilled in theart.

For example, rather than using a reticle imaging lens which occupies afull field of view of the rangefinder, a narrow horizontal section ofthe lens may be employed which occupies but a fraction of therangefinder field angle in the vertical dimension. The reticle itselfmay be of any desired configuration e.g., in photographic cameraembodiments the reticle might define a frame for delimiting the portionof the field recordable by the camera.

Therefore, because certain changes may be made in the above-describedapparatus without departing from the true spirit and scope of theinvention herein involved, it is intended that the subject matter of theabove depiction shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:

1. A binocular rangefinder, comprising:

reticle means; 1

optical means, including imaging means, for projecting a virtual imageof said reticle means a predetermined image distance into a field; and

means for defining spaced left and right eye view locations and forintercepting different portions of common imaging wavefronts definingsaid virtual reticle image, the said image distance of the reticle imagebeing identifiable with the range of objects in the field which appear,to an observer looking into the field through said eye view locations,to be situated in the vicinity of the reticle image.

2. A binocular rangefinder, comprising:

reticle means;

optical means, including imaging means, for projecting a virtual imageof said reticle means a predetermined image distance into a field, saidimaging means includes a par tially reflective positive powercatoptriclens; and

means defining spaced left and right eye view locations interceptingdifferent portions of common imaging wavefronts defining said virtualreticle image, said partially reflective positive catoptric lens beingdisposed across at least a portion of the field'viewable from said leftand right eye view locations, the said image distance of the reticleimage being identifiable with the range of objects in the field whichappear, to an observer looking into the field through said eye viewlocations, to be situated in the vicinity of the reticle image.

3. The apparatus defined by claim 2 wherein said optical means includesrelay means for forming a first image of said reticle means behind andat most a focal length away from said lens for projection by said lensinto the field.

4. The apparatus defined by claim 3 wherein said reticle means isdisposed substantially at said lens and on the optical axis thereof, andwherein said relay means includes a mirror disposed behind saidcatoptric lens for forming said first reticle image.

5. The apparatus defined by claim 4 wherein said reticle means comprisesa mask on a central portion of said catoptric lens, said mask definingwindow means the boundaries of which determine the configuration of saidreticle image.'

6. The apparatus defined by claim 5 including adjustable control meansfor varying the depthwise position of said projected reticle image, saidcontrol means including means coupled to said mirror for adjusting theaxial position of said mirror and thereby the spacing of said firstreticle image from said lens.

7. The apparatus defined by claim 1 wherein said reticle means has asignificant dimension in a direction parallel to the principle axis ofsaid imaging means such that said projected virtual image of saidreticle has a visually perceptible depthwise extension.

8. In a photographic camera the combination comprising: a variable focusobjective lens and focus control means operable to change the focus ofsaid lens; a binocular rangefinder comprising:

reticle means, optical means including imaging means for projecting avirtual image ofsaid reticle means a predetermined image distance into afield, and

means defining spaced left and right eye view locations interceptingdifferent portions of common imaging wavefronts defining said virtualreticle'image; adjustable means for varying the distance said reticleimage is projected; and coupling means coupling said focus control meansand said adjustable means such that adjustment of said focus controlmeans until said reticle image appears to lie in the vicinity of anobject'to be photographed causes said objective lens to be focused onsaid object. 9. Theapparatus defined by claim 8 wherein said imagingmeans includes a partially reflective positive power catoptric lensdisposed across at least a portion of the field viewable from said leftand right eye view locations.

l0. The apparatus defined by claim 9 wherein said optical means includesrelay means for forming a first image of said reticle means behind andat most a focal length away from said lens for projection by said lensinto the field.

11. The apparatus defined by claim 10 wherein said reticle means isdisposed substantially at said lens and on the optical axis thereof, andwherein said relay means includes a mirror disposed behind saidcatoptric lens for forming said first reticle image.

' 12. The apparatus defined by claim 11 wherein said reticle meanscomprises a mask on a central portion of said catoptric lens, said maskdefining window meansthe boundaries of which determine the configurationof said reticle image.

13. The apparatus defined' by claim 11 wherein said coupling means isconnected to'said mirror to effect axial movement thereof in response toadjustments of said-focus control means, movement of said mirroreffecting an axial movement of said first reticle to said lens and thusa depthwise movement of said projected image.

14. The apparatus defined by claim 1 wherein said imaging means includesa positive power dioptric lens, and wherein said optical means includesa partially reflective mirror disposed across at least a'portion of thefield viewable from said left and right eye view locations forreflecting said imaging wavefronts defining said virtual reticle imageto said eye view locations.

15. The apparatus defined by claim 14 wherein said reticle means has asignificant dimension in a direction parallel to the.

principle axis of said imaging means such that said projected virtualimage of said reticle has a visually perceptible depthwise extension.

16. A stereo rangefinder, comprising: means defining left and right eyeview locations; reticle means positioned on an axis extending into therangefinder field along a locus passing equidistant from said left andright eye view locations; a mirror disposed behind said reticle means onsaid axis for forming a first virtual image of said reticle means;positive power partially reflective catoptric lens means positionedforwardly of said mirror and at most a focal length away from said'firstimage of said reticle means forming in the field of the rangefinder asecond virtual image of said reticle means; and

means for varying the depthwise position of said second image in thefield, said means including means coupled to said mirror for moving saidfirst mirror so as to vary the spacing of said first image from saidcatoptric lens means and thus the depthwise position in the field ofsaid second image of said reticle means.

17. A stereo rangefinder, comprising:

means defining left and right eye view locations; reticle meanspositioned on an axis extending into the rangefinder field along alocuspassing equidistant from said left and right eye view locations;

a mirror disposed behind said reticle means on said axis for forming afirst virtual image of said reticle means;

first and second positive power catoptric lens elements respectivelyaligned with said first and second eye view locations and each receivingobject rays from said first image for conjointly forming in the field ofthe rangefinder a virtual image of said first image;

means for varying the depth-wise position of said second image in thefield, said means including means coupled to said mirror for moving saidfirst mirror so as to vary the spacing of said first image from saidcatoptric lens elements and thus the depth-wise position in the field ofsaid second image of said reticle means.

18. A rangefinding method, comprising:

projecting into a field a visible virtual image of a reticle;

binocularly observing the image from spaced left and right eye viewlocations interceptingdifferent portions of common imaging wavefrontsdefining the said virtual reticle image;

adjusting the image distance of the reticle image until the imageappears to generally correspond in spatial position with an object inthe field whose range is sought; and

utilizing the image distance when said correspondence is established asan indication of the range of the object.

19. A binocular rangefinder functioning on the principle ofcorrespondence between two visual convergence angles to determine thedistance to a subject, a first of said convergence angles fusing a righteye image and left eye image of said subject, and a second of saidconvergence angles fusing a right eye image and left eye image of areticle, said binocular rangefinder comprising:

concave mirror means with at least one partially transparent reflectingsurface with a predetermined focal length, said concave mirror meanshaving a right viewing portion to provide said right eye images and aleft viewing portion to provide said left eye images, said portionshaving a generatrix common to both and a fixed orientation with respectto each other; and

reticle means equally spaced from said right viewing portion and fromsaid left viewing portion by optical paths of equal length and viewableby reflection from the concave surface of said concave mirror means.

20. The binocular rangefinder described in claim 19, further comprisingmeans for varying the length of said optical paths 21. The binocularrangefinder described in claim 19, further comprising:

planar mirror means, opposite said concave mirror means,

for reflecting at least one image of said reticle to said concavesurface.

22. The binocular rangefinder described in claim 19, wherein saidoptical paths have lengths not exceeding said predetermined focallength.

1. A binocular rangefinder, comprising: reticle means; optical means,including imaging means, for projecting a virtual image of said reticlemeans a predetermined image distance into a field; and means fordefining spaced left and right eye view locations and for interceptingdifferent portions of common imaging wavefronts defining said virtualreticle image, the said image distance of the reticle image beingidentifiable with the range of objects in the field which appear, to anobserver looking into the field through said eye view locations, to besituated in the vicinity of the reticle image.
 2. A binocularrangefinder, comprising: reticle means; optical means, including imagingmeans, for projecting a virtual image of said reticle means apredetermined image distance into a field, said imaging means includes apartially reflective positive power catoptric lens; and means definingspaced left and right eye view locations intercepting different portionsof common imaging wavefronts defining said virtual reticle image, saidpartially reflectiVe positive catoptric lens being disposed across atleast a portion of the field viewable from said left and right eye viewlocations, the said image distance of the reticle image beingidentifiable with the range of objects in the field which appear, to anobserver looking into the field through said eye view locations, to besituated in the vicinity of the reticle image.
 3. The apparatus definedby claim 2 wherein said optical means includes relay means for forming afirst image of said reticle means behind and at most a focal length awayfrom said lens for projection by said lens into the field.
 4. Theapparatus defined by claim 3 wherein said reticle means is disposedsubstantially at said lens and on the optical axis thereof, and whereinsaid relay means includes a mirror disposed behind said catoptric lensfor forming said first reticle image.
 5. The apparatus defined by claim4 wherein said reticle means comprises a mask on a central portion ofsaid catoptric lens, said mask defining window means the boundaries ofwhich determine the configuration of said reticle image.
 6. Theapparatus defined by claim 5 including adjustable control means forvarying the depthwise position of said projected reticle image, saidcontrol means including means coupled to said mirror for adjusting theaxial position of said mirror and thereby the spacing of said firstreticle image from said lens.
 7. The apparatus defined by claim 1wherein said reticle means has a significant dimension in a directionparallel to the principle axis of said imaging means such that saidprojected virtual image of said reticle has a visually perceptibledepthwise extension.
 8. In a photographic camera the combinationcomprising: a variable focus objective lens and focus control meansoperable to change the focus of said lens; a binocular rangefindercomprising: reticle means, optical means including imaging means forprojecting a virtual image of said reticle means a predetermined imagedistance into a field, and means defining spaced left and right eye viewlocations intercepting different portions of common imaging wavefrontsdefining said virtual reticle image; adjustable means for varying thedistance said reticle image is projected; and coupling means couplingsaid focus control means and said adjustable means such that adjustmentof said focus control means until said reticle image appears to lie inthe vicinity of an object to be photographed causes said objective lensto be focused on said object.
 9. The apparatus defined by claim 8wherein said imaging means includes a partially reflective positivepower catoptric lens disposed across at least a portion of the fieldviewable from said left and right eye view locations.
 10. The apparatusdefined by claim 9 wherein said optical means includes relay means forforming a first image of said reticle means behind and at most a focallength away from said lens for projection by said lens into the field.11. The apparatus defined by claim 10 wherein said reticle means isdisposed substantially at said lens and on the optical axis thereof, andwherein said relay means includes a mirror disposed behind saidcatoptric lens for forming said first reticle image.
 12. The apparatusdefined by claim 11 wherein said reticle means comprises a mask on acentral portion of said catoptric lens, said mask defining window meansthe boundaries of which determine the configuration of said reticleimage.
 13. The apparatus defined by claim 11 wherein said coupling meansis connected to said mirror to effect axial movement thereof in responseto adjustments of said focus control means, movement of said mirroreffecting an axial movement of said first reticle to said lens and thusa depth-wise movement of said projected image.
 14. The apparatus definedby claim 1 wherein said imaging means includes a positive power dioptriclens, and wherein said optical means includes a partially reflectivemirror disposed Across at least a portion of the field viewable fromsaid left and right eye view locations for reflecting said imagingwavefronts defining said virtual reticle image to said eye viewlocations.
 15. The apparatus defined by claim 14 wherein said reticlemeans has a significant dimension in a direction parallel to theprinciple axis of said imaging means such that said projected virtualimage of said reticle has a visually perceptible depth-wise extension.16. A stereo rangefinder, comprising: means defining left and right eyeview locations; reticle means positioned on an axis extending into therangefinder field along a locus passing equidistant from said left andright eye view locations; a mirror disposed behind said reticle means onsaid axis for forming a first virtual image of said reticle means;positive power partially reflective catoptric lens means positionedforwardly of said mirror and at most a focal length away from said firstimage of said reticle means forming in the field of the rangefinder asecond virtual image of said reticle means; and means for varying thedepthwise position of said second image in the field, said meansincluding means coupled to said mirror for moving said first mirror soas to vary the spacing of said first image from said catoptric lensmeans and thus the depthwise position in the field of said second imageof said reticle means.
 17. A stereo rangefinder, comprising: meansdefining left and right eye view locations; reticle means positioned onan axis extending into the rangefinder field along a locus passingequidistant from said left and right eye view locations; a mirrordisposed behind said reticle means on said axis for forming a firstvirtual image of said reticle means; first and second positive powercatoptric lens elements respectively aligned with said first and secondeye view locations and each receiving object rays from said first imagefor conjointly forming in the field of the rangefinder a virtual imageof said first image; means for varying the depth-wise position of saidsecond image in the field, said means including means coupled to saidmirror for moving said first mirror so as to vary the spacing of saidfirst image from said catoptric lens elements and thus the depth-wiseposition in the field of said second image of said reticle means.
 18. Arangefinding method, comprising: projecting into a field a visiblevirtual image of a reticle; binocularly observing the image from spacedleft and right eye view locations intercepting different portions ofcommon imaging wavefronts defining the said virtual reticle image;adjusting the image distance of the reticle image until the imageappears to generally correspond in spatial position with an object inthe field whose range is sought; and utilizing the image distance whensaid correspondence is established as an indication of the range of theobject.
 19. A binocular rangefinder functioning on the principle ofcorrespondence between two visual convergence angles to determine thedistance to a subject, a first of said convergence angles fusing a righteye image and left eye image of said subject, and a second of saidconvergence angles fusing a right eye image and left eye image of areticle, said binocular rangefinder comprising: concave mirror meanswith at least one partially transparent reflecting surface with apredetermined focal length, said concave mirror means having a rightviewing portion to provide said right eye images and a left viewingportion to provide said left eye images, said portions having ageneratrix common to both and a fixed orientation with respect to eachother; and reticle means equally spaced from said right viewing portionand from said left viewing portion by optical paths of equal length andviewable by reflection from the concave surface of said concave mirrormeans.
 20. The binocular rangefinder described in claim 19, furthercompriSing means for varying the length of said optical paths
 21. Thebinocular rangefinder described in claim 19, further comprising: planarmirror means, opposite said concave mirror means, for reflecting atleast one image of said reticle to said concave surface.
 22. Thebinocular rangefinder described in claim 19, wherein said optical pathshave lengths not exceeding said predetermined focal length.